1. Field of the Invention
The present invention relates to the disposition of silicon carbide on semiconductor devices, and more particularly, to a method for forming such silicon carbide deposit containing a minimum amount of oxygen.
2. Description of the Prior Art
Silicon carbide formed by plasma-enhanced chemical vapor deposition (PECVD), possessing a low dielectric constant and high resistivity, has become a potential substitute for silicon nitride in semiconductor IC (Integrated Circuits) fabrication. As device technology leads to smaller and smaller geometries, the development of the silicon carbide film is believed to be one solution for resolving possible RC delay phenomenon during IC fabrication.
A film of PECVD silicon carbide is typically deposited from gaseous organosilicon such as silane/methane, trimethylsilane or tertramethylsilane. The deposition may be carried out in a single step or in multiple steps. The PECVD film generally contains large amounts of bonded hydrogen in the form of Sixe2x80x94H and Cxe2x80x94H, and the composition of which is thus represented as SiCxHy. PECVD carbide deposit is similar to a nitride film in the way that they are both impermeable to oxygen and water. Such film of appropriate thickness can be used as a mask to block oxidation of the substrate underneath and will oxidize very slowly at the surface. In addition, the carbide material is found to exhibit excellent insulating properties, such as low dielectric constant (in the range of 4-5) and high resistivity towards copper diffusion. As a result, a PECVD silicon carbide film is an excellent choice other than nitride for making insulators such as copper barrier or hardmask during IC fabrication.
However, the deposition of a typical PECVD silicon carbide film suffers from one major difficulty, that is, the purification of the carbide composition. The conventionally-formed silicon carbide is found to contain relatively high amounts of oxygen, typically in thousands of ppm level, which is more than ten times the typical oxygen content in a nitride deposit. Such a high content of oxygen atoms could constitute up to 3% (generally between 0.1-0.5%) of the deposited carbide composition and thus greatly restricts the applicability of the insulator. Taking copper barriers for example, the composed barrier material is expected to appropriately inhibit copper diffusion, yet an oxygen-rich substance such as the conventional PECVD silicon carbide would adversely enhance copper oxidation and thus largely increase resistance at the copper side. Similarly, application of the oxygen-rich material for a hardmask is unfavorable, especially for hardmasking low-k interlevel insulators. An oxygen-rich hardmask tends to enhance oxidation of the substrate underneath and results in an inexpedient increase in the dielectric constant of the substrate material.
Therefore, a need exists for a process which can effectively reduce the oxygen content of the silicon carbide deposit.
In accordance with the present invention, a method is provided to reduce the oxygen content of PECVD silicon carbide deposit. The invention modifies the conventional deposition process to be compatible with production-level IC processing. In accordance with an aspect of the present invention, even if the silicon carbide film is inevitably formed containing oxygen as impurities, the oxygen content is as low as that in a comparable silicon nitride layer.
In accordance with the present invention, the oxygen reduction is acheived by an additional plasma treatment following the film deposition step. The plasma treatment is performed in-situ with a PECVD silicon carbide process, therefore, no additional fabrication facilities are necessary to practice the present innovation as compared to prior art silicon carbide processes. More specifically, every oxygen removal treatment is performed within one PECVD reaction chamber (the one used for depositing silicon carbide) by introducing appropriate plasma of sufficient energy into the chamber. The plasma reacts with the film-containing oxygen and carries the reaction product away from the carbide deposit. Tests show that the additional use of such treatment can cut down the oxygen content by an order of a decimal. In addition, the disclosed method is found to improve breakdown voltage, leakage current and copper diffusion resistance of the silicon carbide deposit.